On-Surface Synthesis of Hexabenzocoronenes via a Retro-Büchner Reaction of Heptaarylcycloheptatrienes.

On-surface synthesis has become a powerful technique for obtaining various nanostructures, mainly by linking small precursor molecules. In contrast, the cleavage of specific moieties from precursor molecules is also an important and promising strategy to increase the variety of products. Here, we report the on-surface synthesis of hexabenzocoronene (HBC) and its hexa-fluorinated analogue (6F-HBC) from the parent heptaphenylcycloheptatrienes, via the systematic extrusion of a phenyl-CH: carbenic fragment and a subsequent cyclodehydrogenation process on a Au(111) substrate. Their structures and electronic properties were investigated in-depth by low-temperature scanning tunneling microscopy with a CO-terminated tip. Annealing at 200 °C triggers the elimination of the phenylcarbenic groups from the cycloheptatriene core, resulting in the formation of both hexaphenylbenzene and its fluorinated analogue. Although the former was transformed into HBC via cyclodehydrogenation at 300 °C, the fluorinated one was thermally desorbed from the surface even at 250 °C. We found that depositing fluorinated heptaphenylcycloheptatriene onto the substrate kept at 300 °C successfully yielded 6F-HBC. The influence of fluorine substitution was identified through molecular assemblies and bond-resolved images as well as dI/dV measurements, all obtained using the same CO-terminated tip. Our findings demonstrate that the retro-Büchner reaction, corresponding to a ring contraction, is an effective strategy to enhance the variety of structures and electronic properties of on-surface synthesized products.